1. Field of the Invention
This invention relates generally to transferring loads between adjacent cast-in-place slabs, and, more particularly, to a system for transferring, across a joint between a first slab and a second slab, a load applied to either slab.
2. Related Art
A concrete floor is typically made up of a series of individual blocks or slabs, as shown in FIG. 1. The same is true for sidewalks, driveways, roads, and the like. Individual slabs provide several advantages including relief of internal stress due to drying shrinkage and thermal movement. Adjacent slabs meet at joints. Joints are typically spaced so that each slab has enough strength to overcome internal stresses that would otherwise cause random stress relief cracks. In practice, slabs should be allowed to move individually but should also be able to transfer loads from one slab to an adjacent slab. Transferring loads between slabs is usually accomplished using dowels, embedded in the two adjacent slabs defining the joint.
U.S. Pat. Nos. 5,005,331, 5,216,862, and 5,487,249 issued to Shaw et al., incorporated herein by reference, disclose tubular dowel receiving pocket formers for use with dowel bars having a circular cross-section.
If circular or square dowels, are misaligned (i.e., not positioned perpendicular to the joint), they can undesirably lock the joint together causing unwanted stresses that could lead to slab failure in the form of cracking. Another shortcoming of square and round dowels is that they typically allow slabs to move only along the longitudinal axis of the dowel. Such restraint of movement in directions other than parallel to the longitudinal axes of dowels may result in slab failure in the form of cracking.
U.S. Pat. No. 4,733,513 issued to Schrader et al., incorporated herein by reference, discloses a dowel bar having a rectangular cross-section and resilient facings attached to the sides of the bar. A shortcoming of prior art dowel bars results from the fact that, under a load, only the first 3-4 inches of each dowel bar is typically used for transferring the load. This creates very high loadings per square inch at the edge of slab, which can result in failure of the concrete below dowel. Such a failure could also occur above dowel.
U.S. Pat. No. 6,354,760 (“the '760 patent”issued to Boxall and Parkes, incorporated herein by reference, discloses a tapered load plate for transferring loads between adjacent concrete slabs. The tapered load plate permits relative movement between slabs in a direction parallel to the longitudinal axis of the joint, while reducing the loading per square inch of the dowel close to the joint. A pocket former embedded within one of the slabs for positioning the load plate is also disclosed.
In the '760 patent, the relative movement of the two adjacent concrete slabs is directly proportional to the extent that the joint between the two slabs opens due to the requirement of a tapered load plate. I.e., the more the joint opens, the more lateral movement is permitted.
Accordingly, there is a need in the art for a load plate system that provides for significant relative movement along the joint between two adjacent concrete slabs where the joint between the slabs opens only enough to overcome the interface friction between the two adjacent concrete slabs.